The present invention relates to electrical connectors for printed circuit board applications. More particularly, the invention relates to the configuration of an electrical conductor contact, a plurality of which are used in a receptacle connector for low force mating with a pin header in printed circuit board applications. The invention also more particularly relates to a receptacle connector housing which houses a plurality of electrical conductor contacts.
Printed circuit boards have become widely used in a plethora of electronic applications. As electrical circuits become increasingly complicated, it is often necessary to provide more than one printed circuit board for an application, with the resulting necessity of employing circuit board electrical connectors to establish electrical connections between the boards. One common means provided in the art for electrically connecting printed circuit boards is the standard two-piece or "post and box" high density connector which is comprised of a pin header having a plurality of 0.025 inch square or round posts in close proximity one to the other, and a receptacle socket connector which is configured with spring contacts which receive the pin header. The pin header is attached or electrically connected to a first printed circuit board, while the receptacle socket connector is electrically connected to the second board.
While complicated applications have led to the use of multiple printed circuit boards, the increasing complexity of the circuits and integrated circuits contained on the printed circuit boards has led to increasingly larger connectors such that pin headers with six hundred posts are now known in the art. Accompanying these large connectors is the problem of permitting the pin header and receptacle connector to mate without an extraordinarily large mating force, the misapplication of which could damage individual posts or connector contacts, making disconnection extremely difficult. Optimally, the pin header should be able to be inserted into and removed from the receptacle connector without causing damage or excessive wear to either the connector contacts or posts. At the same time, the connection between the posts and the connector contacts must be secure to provide a good electrical connection. Generally, the greater the normal force (defined as the force exerted in a direction perpendicular to the direction of insertion of the post) which a receptacle connector spring contact exerts on the conducting post, the better is the electrical connection which results. However, the greater the force, the greater the possibility of post damage or connector contact wear. Thus, minimum normal forces which will provide a desired quality of electrical connection while reducing the chances of damage are often determined when designing connectors.
Minimum normal or "contact" forces provide the connector designer with the minimum total force required to mate the post and box contacts, as the mating force is related to the normal force through a friction coefficient. Such a minimum mating force is realized, however, only in the ideal situation where no manufacturing tolerances are involved. Where the posts have manufacturing thickness tolerances, and the spring contacts have spring rate tolerances, those skilled in the art will understand that the effect of such tolerances provides another force designated as the "maximum mating force" which is a result of insuring that the minimum normal force is provided to each post and spring connection. It is thus clearly desirable to design a connector whose minimum and maximum mating forces are similar and small.
It has been recognized that by providing spring contacts with small spring rates (also called "spring constants" and defined in terms of gms/mil deflection) and permitting large spring deflections, the "small and similar" requirements can be met. Thus, if two cantilever springs with relatively small spring rates of 4 grams/mil are provided, and the springs are in contact but not preloaded against each other and are expected to be deflected by a 0.025 post, a normal minimum force of 50 grams per spring (4 grams.times.12.5 mils) is provided. If the post manufacturing tolerance is .+-.1 mil, a minimum spring rate of 4.17 gms/mil (50 gms/12 mils) must be provided to insure the 50 gram minimum normal force. With such a minimum spring rate, and a spring rate tolerance of .+-.1 grams/mil, the maximum normal force would be 80.2 grams per spring (6.17 grams/mil.times.13 mils). Additionally, the springs have a manufacturing tolerance on their positions with respect to each other. If the gap between springs is 3 mils.+-.3 mils, the minimum spring rate required would be 5.55 gms/mil (50 gms/9 mils) with the same post manufacturing tolerance, and the maximum normal force would be 94.4 grams (7.55 gms/mil.times.13 mils). On the other hand, if the springs were provided with a spring rate of 50 grams/mil, and the springs were located 20.+-.3 mils apart, the deflection by a 0.025 post also would provide a minimum force of 50 grams per spring (50 grams/mil.times.1 mil). However, in this situation, even without a post manufacturing tolerance or a spring rate tolerance, a much larger maximum normal force of 200 grams would result (50 grams/mil.times.4 mils). Thus, it is evident that to provide acceptable minimum and maximum normal forces, low spring rates and large spring deflections are desirable.
In order to provide large spring deflections with a 0.025 pin and low maximum mating forces, the contact springs have been placed in close proximity one to the other by those skilled in the art. The difficulties with providing extremely small gaps or no gaps between spring contacts include the facts that the springs and/or the posts are prone to damage when forced mating occurs, and that the metal plating of the spring contacts either must be accomplished before forming occurs (in the case of no gap) or excess precious metals must be used in the plating process if plating occurs after forming. Excess precious metals are required to plate the contact surface in the case of a small gap because the small gap does not permit the free motion of the plating fluid around the contact surface, and plating metal does not easily deposit onto the desired location. Thus, in order to get the required contact surface plating where only a small gap exists, the open surfaces get more plating than is required, i.e. excess precious metals are used.
To alleviate the problem of damage during mating, a technique called "preloading" has been used. Preloading permits large deflection without damage during mating by taking the formed springs, and separating them with a nonconductive material such as plastic. When the mating post element enters the now enlarged gap between the spring contacts, damage is less likely to occur because the tapered post is easily accepted by the separated springs. When the post is inserted further into the connector, the post separates the springs further, as the post diameter is greater than the spring contact gap provided by the plastic preloading elements. Thus, in the ultimate position, the spring contacts act upon the post and the entire mating force is applied to the post rather than to the plastic.
While the techniques of preloading and providing low spring rates with large deflections have been advances in the art, the known uses of these techniques have not solved all of the problems relating to mating large pin headers to receptacle connectors. Thus, it is still desirable to design a receptacle connector which can mate with a lower mating force than those provided in the art while maintaining a desired minimum normal force. Moreover, it would be advantageous to overcome the costly requirement of using added amounts of precious metals in the plating process while still providing springs which will undergo large deflection.